Newspaper stackers are well known for creating signature (i.e. newspaper) bundles of precise count for subsequent tying and delivery. Conventional stackers accept signatures arranged in a "shingle" or imbricated fashion delivered from a newspaper press typically at rates as high as eighty thousand per hour or greater. The stacker is provided with intercept means for intercepting the signature stream and collecting signatures upon a stacking platform or "bucket", the signatures being accumulated thereon until the desired count is reached at which time an upstream bucket is caused to intercept the signature stream and begin collection of the next stack.
To meet present requirements, stackers must be capable of stacking any desired count of signatures and further be capable of forming stacks of signatures of differing amounts wherein each successive stack may be a count different from the proceeding downstream stack.
Extreme applications exist wherein the difference between the count of a substantially completed stack and the next stack to be formed is quite significant and further wherein it is desired to be capable of forming stacks of extremely small count. In conventional stackers, there is provided a single motor for driving a pair of drive chains. Chain driven buckets arranged at spaced intervals along the chains pass the downstream end of an infeed conveyor section as the chain drive is operated to cause the newspaper stream to be intercepted. Counting means is typically provided in the stacker infeed conveyor section for counting the signatures. When a predetermined count is reached, the bucket immediately behind the bucket receiving newspapers is moved to the intercept position causing subsequent newspapers to be collected on the bucket which was just moved to the intercept position. Since all of the buckets are mechanically linked to the drive chains, all of the buckets are driven at the same operating speed imposing severe limitations upon bundle size for the reason that once a bucket is moved to the intercept position, the bucket is driven through the stacking region at normal stacking speed. Thus, the bucket immediately behind the bucket which has been moved to the intercept position likewise moves to the latched or "home" position at normal stacking speed. If the bucket in the stacking region receives the predetermined number of newspapers before the next bucket coupled to the drive chain reaches the latched position, then the next bucket in line will not be provided with the necessary amount of acceleration required to cleanly intercept the newspaper stream and thereby assure an accurate count. In addition, when the stacker receives signatures at very high press speeds, the feed rate of signatures being stacked may be so great as to create a stack of signatures larger than the desired number before the next bucket may be moved to the intercept position, likewise causing an error in signature count. Due to these factors, conventional stackers have the disadvantages of being limited as to the smallest bundle which can be formed by the stacker and also have an upper limit as to the number of newspapers per unit time which can be fed to the stacker for rapidly forming bundles of an exact predetermined count.
In order to overcome these limitations, dual drive stackers have been developed. Such dual drive stackers are described, for example, in U.S. Pat. Nos. 3,479,932 and 3,526,170 in which first and second drive chains are independently driven by either independent motors or a single motor and appropriate coupling and speed control means including clutches and the like. In U.S. Pat. No. 3,479,932 the drive chains are driven at either a normal stacking speed or at a high speed greater than the normal stacking speed. Each set of chains is provided with at least one stacking bucket so that while one bucket is moving through the stacking region at normal stacking speed, the other bucket associated with the other set of chains is moved at high speed to the home or intercept ready position in readiness for receiving and collecting the next signature stack.
U.S. Pat. No. 3,526,170 provides first and second variable speed motors and a complicated mechanical arrangement of cams and cooperating cam switches for altering the motor speed at various locations about the path traversed by each stacking bucket. Also the system requires mechanical blocking cams to prevent collision between buckets driven by different chain drives.
The prior art dual drive stackers as represented by the aforementioned U.S. patents utilize independent chain drives which are arranged on common shafts, requiring a complicated arrangement of mounting bearings rendering it a practical impossibility to properly independently tension each of the drive chains. In addition, the systems require mechanical latching means at the intercept ready position and also lack means for substantially instantaneously regulating the bucket operating speed to accommodate any changes whether they be from stack-to-stack or signature-to-signature during the formation of a stack.